BOULDER—Even if all greenhouse gases had been stabilized in the year
2000, we would still be committed to a warmer Earth and greater sea level
rise in the present century, according to a new study by a team of climate
modelers at the National Center for Atmospheric Research (NCAR). The
findings are published in this week's issue of the journal Science.

The modeling study quantifies the relative rates of sea level rise and
global temperature increase that we are already committed to in the 21st
century. Even if atmospheric levels of greenhouse gases leveled off,
globally averaged surface air temperatures would rise about a half degree
Celsius (one degree Fahrenheit) and global sea levels would rise another
11 centimeters (4 inches) from thermal expansion alone by 2100.

“Many people don’t realize we are committed right now to
a significant amount of global warming and sea level rise because of
the greenhouse gases we have already put into the atmosphere,” says
lead author Gerald Meehl. “Even if we stabilize greenhouse gas
concentrations, the climate will continue to warm, and there will be
proportionately even more sea level rise. The longer we wait, the more
climate change we are committed to in the future.”

Coastal population centers at low elevations like
New Orleans, Key West, and other spots around the world, are
vulnerable to sea level rise. Florida's Key West floods
routinely in springtime when tides pulled higher by the new or full
moon reach 1.7 feet above mean sea level. (Photo courtesy National
Weather Service Forecast Office, Key West; NOAA
Photo Library.)

The half-degree temperature rise is similar to that observed at the
end of the 20th century, but the projected sea level rise is more than
twice the 3-inch (5-centimeter) rise that occurred during the latter
half of the previous century. These numbers do not take into account
fresh water from melting ice sheets and glaciers, which could at least
double the sea level rise caused by thermal expansion alone.

The North Atlantic thermohaline circulation, which currently warms Europe
by transporting heat from the tropics, weakens in the models. Even so,
Europe heats up with the rest of the planet because of the overwhelming
effect of greenhouse gases.

Though temperature rise shows signs of leveling off 100 years after
stabilization in the study, ocean waters continue to warm and expand,
causing global sea level to rise unabated.

The paper concludes with a cogent statement by Meehl: "With the
ongoing increase in concentrations of GHGs [greenhouse gases], every
day we commit to more climate change in the future. When and how we stabilize
concentrations will dictate, on the time scale of a century or so, how
much more warming we will experience. But we are already committed to
ongoing large sea level rise, even if concentrations of GHGs could be
stabilized."

The inevitability of the climate changes described in the study is the
result of thermal inertia, mainly from the oceans, and the long lifetime
of carbon dioxide and other greenhouse gases in the atmosphere. Thermal
inertia refers to the process by which water heats and cools more slowly
than air because it is denser than air.

The new study is the first to quantify future committed climate change
using "coupled" global three-dimensional climate models. Coupled
models link major components of Earth's climate in ways that allow them
to interact with each other. Meehl and his NCAR colleagues ran the same
scenario a number of times and averaged the results to create ensemble
simulations from each of two global climate models. Then they compared
the results from each model.

The scientists also compared possible climate scenarios in the two models
during the 21st century in which greenhouse gases continue to build in
the atmosphere at low, moderate, or high rates. The worst-case scenario
projects an average temperature rise of 3.5°C (6.3°F) and sea
level rise from thermal expansion of 30 centimeters (12 inches) by 2100.
All scenarios analyzed in the study will be assessed by international
teams of scientists for the next report by the Intergovernmental Panel
on Climate Change, due out in 2007.

The NCAR team used the Parallel Climate Model (PCM), developed by NCAR
and the Department of Energy, and the new Community Climate System Model
(Version 3). The CCSM3 was developed at NCAR with input from university
and federal climate scientists around the country and principal funding
from the National Science Foundation (NCAR’s primary sponsor) and
the Department of Energy. The CCSM3 shows slightly higher temperature
rise and sea level rise from thermal expansion and greater weakening
of the thermohaline circulation in the North Atlantic. Otherwise, the
results from the two models are similar. The models were run on supercomputers
at NCAR and several DOE labs and on the Earth Simulator in Japan.

Another paper in this week's issue of Science, "The Climate Change Commitment," by NCAR scientist Tom Wigley, calculates a continued rise in temperatures and sea level out to the year 2400, using a different computer model.

The National Center for Atmospheric Research and UCAR Office of Programs are operated by UCAR under the sponsorship of the National Science Foundation and other agencies. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors.